[WIP] Refactor unit handling

- Remove Unit model
- Use ruby-units library instead
[WIP] OAuth2 support (thx to Omniauth), refs #15

- Added username field to User model
- User has_many Authentications, each for every connected Omniauth provider
- Logging in is possible now with username, email or one of the supported Omniauth providers
- Added devise views to the project
Update Gemfile
Use localhost SMTP for sending emails